My understanding is that the GY6 is only found in Chinese scooters, and only 150cc. The Buddy is built by PGO in Taiwan, and is likely to be completely different. If the damage was cause by running it without final drive oil, it may not be so complicated after all. Probably does not involve the engine or even the variator. Most everyone on the Zuma forum recommend UNDERFILLING the final drive on Zumas and Vinos. I slightly overfill mine. It is vented so it won't blow a seal. And I have never had any oil blown out the vent.

Often repeated, especially on the internet, but nobody has ever come up with anything from Honda referring to a GY6, or parts fiches or service manuals about this motor. AFAIK, Honda produces service manuals and parts fiches for pretty much everything they make.
The first examples of the 50cc version I have seen came on the Kymco Filly. That is the only non-Chinese one I have seen that uses a 669 belt, and it has a design flaw, corrected in later Kymco versions but still in the Chinese versions, that is the primary reason you don't see many high mileage Chinese scooters that have not had the top end replaced. When China banned two strokes on their own roads, the 150cc GY6 was rushed into production, and there is an additional design flaw in the version the Chinese chose to copy which causes abnormally high crankcase pressure to build up.

Often repeated, especially on the internet, but nobody has ever come up with anything from Honda referring to a GY6, or parts fiches or service manuals about this motor. AFAIK, Honda produces service manuals and parts fiches for pretty much everything they make.
The first examples of the 50cc version I have seen came on the Kymco Filly. That is the only non-Chinese one I have seen that uses a 669 belt, and it has a design flaw, corrected in later Kymco versions but still in the Chinese versions, that is the primary reason you don't see many high mileage Chinese scooters that have not had the top end replaced. When China banned two strokes on their own roads, the 150cc GY6 was rushed into production, and there is an additional design flaw in the version the Chinese chose to copy which causes abnormally high crankcase pressure to build up.

So is the GY6 a Kymco design, made into a 150cc by the Chinese? Aside from the crankcase venting problem (which the earlier Honda GET engines used in the Metropolitan and Ruckus also had) what is the other flaw in the GY6? Do current Chinese GY6 scooters still have these problems?

So is the GY6 a Kymco design, made into a 150cc by the Chinese? Aside from the crankcase venting problem (which the earlier Honda GET engines used in the Metropolitan and Ruckus also had) what is the other flaw in the GY6? Do current Chinese GY6 scooters still have these problems?

The 150 was also being made by the Taiwanese, but it was not quite perfected when the Chinese decided to "borrow" it. The other problem is the design of the camtower on the 50cc head. It is still being used on the Chinese product today.